With an aim of saving energy (reducing fuel consumption) and reducing the amount of the exhaust gas (carbon dioxide, nitrogen oxide, particulate matter, and the like) which is discharged from the engine and becomes a cause of the environmental load, hybrid construction machinery is publicly known which includes a motor generator in addition to an engine as a power source. In order to save energy and to reduce the amount of the exhaust gas that becomes a cause of the environmental load, it is particularly important to operate an engine at a proper operating condition. In hybrid construction machinery, the engine can be operated under a proper operation state by controlling two power sources of the engine and the motor generator so that the power required for a work is properly distributed to hydraulic pumps.
With respect to a method of properly distributing the power of the engine and the motor generator, technologies shown in Patent Literature 1 and Patent Literature 2 are publicly known. In Patent Literature 1, the pump requiring power that is the power required for a hydraulic pump and the engine maximum power that corresponds to the target rotational speed of the engine determined based on the accelerator signal are compared to each other, and excess or deficiency of the engine maximum power relative to the pump requiring power is calculated as the motor generator power. Further, by correcting the motor generator power according to the deviation relative to the engine speed caused by the difference between the calculation value and the actual value of the pump requiring power, power distribution between the engine and the motor generator is achieved.
Further, in Patent Literature 2, a method is shown which determines the power distribution between the engine and the motor generator based on the actuator requiring power which is the power required by the actuator, the charging power according to the charging amount set in the direction of keeping the charging amount of the electrical storage apparatus within a constant range, and the engine power set according to the generating power and the charging amount of the electrical storage apparatus.